Help with blurry pictures

[gdarch]

hello all, i recently took some pictures at a friends graduation, but sadly half the pictures i took came out blurry. I didn't want the background to black out so i set me camera to Aperture mode, and used the rear sync flash (although i don't have another flash so it was the popup flash). can anyone please help with what i did wrong so i dont make that mistake again.

http://farm3.static.flickr.com/2169/2545399429_936d1ca419.jpg?v=0

heres a close up...
http://farm3.static.flickr.com/2179/2546226780_acc1c9b7ec.jpg?v=0

iso 800
f 4.2
1/3 sec
flash in rear sync mode

[jpl]

This one is easy. Your shutter speed is WAY too slow at 1/3 for a handheld shot. 1/60 is generally the slowest shutter speed you can shot haldheld without blur. Of course it depends on how steady your hands are, if you brace yourself and your camera especially if it has VR or IS.

Shot in P mode until you get the hang of what settings the camera chooses in different lighting situations.

[pathfinder]

In Av mode, the camera will try to expose the background with ambient light correctly. This then requires the very long shutter speed of 1/3rd of a sec. You cannot handhold this shutter speed.

Shoot in Manual mode with your flash in iTTL or whatever Nikon calls it. You will have better success,